1. Field of the Invention
The invention relates to communication systems, particularly systems where a plurality of receivers and transmitters are coupled to a common bus or cable.
2. Prior Art
In some communications systems, it is desirable to couple many transmitters and receivers to a common cable or bus. For example, a single coaxial cable may be routed to several buildings in a complex, with several transmitters and receivers (or transceivers) at each building coupled to the cable. Data is then transmitted from a transmitter to a selected (addressed) receiver or group of receivers. The rate at which data is transferred is limited by the bandwidth of the entire cable. A data rate of 3 megabits per second for a mile of cable is typical in these systems.
In common cable systems of the type described where long cable lengths are used, the delay time for a signal to travel end to end along the cable can become significant. For example, referring to FIG. 10, transmitter A begins to transmit at time t.sub.o. Before A's signal can reach transmitter B, B not sensing any cable use also begins to transmit at time t.sub.1. A collision of signals results, and thus some collision avoidance mechanism is needed on the cable to avoid chaos.
Collision avoidance systems to date, such as used in "Ethernet", (U.S. Pat. No. 4,063,220) have relied on circuitry which after identifying a collision aborts the data transmission and waits a random time before retransmitting. The retransmit delay time is weighted given the number of collisions detected. This system is simply based on the premise that eventually one transmitter will gain control of the cable and complete its message. The longer the cable and further apart each respective transmitter and receiver is, the more likely multiple collisions will occur and the longer it takes for any transmitting unit to obtain control over the cable.
The Ethernet collision detector circuit relies on an exclusive-Or gate, the inputs of which are connected to the transmitter input and receiver output. By inserting the appropriate time delay on the transmitter input of the gate, and matching the delay time to the switching delays of the transmitter and receiver, the exclusive-Or gate will receive nearly identical signals and the output will thus remain low. If signals other than those originating from the particular transmitter are received, the gate will no longer receive the same signals and the gate output will be high, thereby indicating a collision. Other circuits associated with an Ethernet system are described in this Patent in order to more fully identify the nature of the present invention.
Thus, in systems such as Ethernet, whether or not any one transmitter will acquire the use of the cable is based essentially on chance. It is not possible to assign to such a system a transmission hierarchy, giving cetain transceivers priority, and until the present invention was developed it was not possible for receivers using a common cable to determine the direction from which a colliding signal arose.
As will be disclosed below, the present invention provides a means whereby each transceiver can detect both the presence and direction of an interfering signal, even while the transceiver is transmitting. In addition, since the direction of the conflicting transmitter can be determined, a left or right hierarchy can be created to give certain units priority in transmitting.